Masteries

Offense:
9

Defense:
0

Utility:
21

If you like to sit back and still be able to harass your enemies, Janna is the right champion for you, she can do massive amounts of damage from far away and still be able to survive even tho she is a bit squishy, more on this in a moment.

Summoners spells:

Clarity: a must have imo, this will keep you on the lane for waaaaaay longer than anyone else you face, most of the time if i'm mid (which you should always try to be) i can actually force the other person to go back, and get his turret to half health just from this. Late game still really amazing just because not only you will be able to stay longer to support your whole team with your shields, but also give them some mana as well (specially with the point in improved clarity on your mastery tree).

Heal: you will probably argue than her ultimate is already a heal, but this will also help you stay in lane longer early game if you are seriously getting harassed, and late game it will let you support your group fights better, it has been the difference between winning a 5v5 instead of having half my team dead.

Other options would probably be Teleport or Ghost.

Early Game: 1-8

If you managed to get the mid lane (again, you should always try to ask your team to let you have it), focus on using your Howling Gale on the minion line, this will give you the edge because you can clear them so much faster than the opposite champion he will have to back off and probably just sit on his turret, the tornado is very easy to dodge by enemy champions if they know when it's coming so don't waste your mana trying to cast it on them, but do try to precast when you know the next wave is coming and maybe also be able to catch the champion off guard and so he walks right into it. If you notice that you're stuck in the mid, you can also ask someone to help you gank mid, make sure to use your shield on them so they do more dmg and don't lose too much health if they have to turret dive for the kill.

Mid Game: 9-14

Remember you should be staying in lane as long as possible, but at this point you will probably go back to get some of your items, if all is going well, you should just keep harassing the mid so that the opposite team has to call 2 people on mid just to keep you off, that way your team can push whichever side is more light on defense, i never usually run up to a lane to help, but rather force the other team to come mid for me, of course be extra careful at this point to use your shield and not be too far in if there's enemies MIA. If there's still only 1 person mid, call one of your fast hitters from your team to help you push that second turret down (if all is going well you will most likely have the first turret down before you even recall for the first time). Remember it's fine to call for help if there's 3+ in the middle, you are STILL squishy if they all focus fire you down.

Late Game: 15-18

You should be playing a more support role at this point, keep your Howling Gale running to clear fast those minion waves or to knock up a champion so your team can catch him, also this is when you need to start using your Zephyr the most, not only for the extra dmg but also to slow them down away from their team/turret. Remember your Eye of the Storm will give an extra dmg boost to whoever you use it on, so if you have enough mana (which you should unless you just finished a 5v5 heavy encounter) try to keep casting it on your team mates to take turrets/inhibitors down faster.

One thing i'll say, don't expect to get the top kills while playing Janna, she's a very good pusher, very good support, and an assist machine XD so you should have plenty of stacks on your soulstealer even without getting the kills. This build should give you overall all the AP and survival you need, but leave the Rylia's Staff as a wild card, which can be replaced by even more survival (Guardian's Angel/Force of Nature) or by a Zhonya's Ring for more AP.